Lithographic apparatus and device manufacturing method

ABSTRACT

A difficulty of contamination interfering with a grid plate positional measurement system is addressed. In one embodiment contamination is prevented from coming into contact with the grating or the sensor. In an embodiment, surface acoustic waves are used to detach contamination from a surface of the grating or sensor.

This application claims priority and benefit under 35 U.S.C. §119(e) toU.S. Provisional Patent Application No. 61/193,487, entitled“Lithographic Apparatus and Device Manufacturing Method”, filed on Dec.3, 2008. The content of that application is incorporated herein in itsentirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The liquid is desirablydistilled water, although another liquid can be used. An embodiment ofthe present invention will be described with reference to liquid.However, a fluid may be suitable, particularly a wetting fluid, anincompressible fluid and/or a fluid with a higher refractive index thanair, desirably a higher refractive index than water. A fluid excludinggas is particularly desired. The point of this is to enable imaging ofsmaller features since the exposure radiation will have a shorterwavelength in the liquid. (The effect of the liquid may also be regardedas increasing the effective numerical aperture (NA) of the system andalso increasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein, or a liquid with a nano-particle suspension (e.g. particleswith a maximum dimension of up to 10 nm). The suspended particles may ormay not have a similar or the same refractive index as the liquid inwhich they are suspended. Other liquids which may be suitable are ahydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueoussolution.

Submersing the substrate or substrate and substrate table in a bath ofliquid (see, for example, U.S. Pat. No. 4,509,852) means that there is alarge body of liquid that must be accelerated during a scanningexposure. This requires additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

In an immersion apparatus, immersion fluid is handled by a fluidhandling system, structure or apparatus. In an embodiment the fluidhandling system may supply immersion fluid and therefore be a fluidsupply system. In an embodiment the fluid handling system may at leastpartly confine immersion fluid and thereby be a fluid confinementsystem. In an embodiment the fluid handling system may provide a barrierto immersion fluid and thereby be a barrier member, such as a fluidconfinement structure. In an embodiment the fluid handling system maycreate or use a flow of gas, for example to help in controlling the flowand/or the position of the immersion fluid. The flow of gas may form aseal to confine the immersion fluid so the fluid handling structure maybe referred to as a seal member; such a seal member may be a fluidconfinement structure. In an embodiment, immersion liquid is used as theimmersion fluid. In that case the fluid handling system may be a liquidhandling system. In reference to the aforementioned description,reference in this paragraph to a feature defined with respect to fluidmay be understood to include a feature defined with respect to liquid.

One of the arrangements proposed is for a liquid supply system toprovide liquid on only a localized area of the substrate and in betweenthe final element of the projection system and the substrate using aliquid confinement system (the substrate generally has a larger surfacearea than the final element of the projection system). One way which hasbeen proposed to arrange for this is disclosed in PCT patent applicationpublication no. WO 99/49504. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet onto the substrate, preferably along thedirection of movement of the substrate relative to the final element,and is removed by at least one outlet after having passed under theprojection system. That is, as the substrate is scanned beneath theelement in a −X direction, liquid is supplied at the +X side of theelement and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet and is taken up onthe other side of the element by outlet which is connected to a lowpressure source. In the illustration of FIG. 2 the liquid is suppliedalong the direction of movement of the substrate W relative to the finalelement, though this does not need to be the case. Various orientationsand numbers of in- and out-lets positioned around the final element arepossible, one example is illustrated in FIG. 3 in which four sets of aninlet with an outlet on either side are provided in a regular patternaround the final element. Note that the direction of flow of the liquidis shown by arrows in FIGS. 2 and 3.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets oneither side of the projection system PS and is removed by a plurality ofdiscrete outlets arranged radially outwardly of the inlets. The inletsand outlets can be arranged in a plate with a hole in its center andthrough which the projection beam is projected. Liquid is supplied byone groove inlet on one side of the projection system PS and removed bya plurality of discrete outlets on the other side of the projectionsystem PS, causing a flow of a thin film of liquid between theprojection system PS and the substrate W. The choice of whichcombination of inlet and outlets to use can depend on the direction ofmovement of the substrate W (the other combination of inlet and outletsbeing inactive). Note that the direction of flow of fluid and of thesubstrate W is shown by arrows in FIG. 4.

In European patent application publication no. EP 1420300 and UnitedStates patent application publication no. US 2004-0136494, the idea of atwin or dual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

PCT patent application publication WO 2005/064405 discloses an all wetarrangement in which the immersion liquid is unconfined. In such asystem the whole top surface of the substrate is covered in liquid. Thismay be advantageous because then the whole top surface of the substrateis exposed to the substantially same conditions. This has an advantagefor temperature control and processing of the substrate. In WO2005/064405, a liquid supply system provides liquid to the gap betweenthe final element of the projection system and the substrate. Thatliquid is allowed to leak over the remainder of the substrate. A barrierat the edge of a substrate table prevents the liquid from escaping sothat it can be removed from the top surface of the substrate table in acontrolled way. Although such a system improves temperature control andprocessing of the substrate, evaporation of the immersion liquid maystill occur. One way of helping to alleviate that problem is describedin United States patent application publication no. US 2006/0119809. Amember is provided which covers the substrate in all positions and whichis arranged to have immersion liquid extending between it and the topsurface of the substrate and/or substrate table which holds thesubstrate.

A type of positional measurement device used in a lithographicapparatus, and in particular in an immersion lithographic apparatus (ofany type), comprises a grating, a radiation source and a sensor. Thegrating and sensor are mounted on the different objects which aremovable relative to one another and whose relative position is desiredto be measured. For example, the grating may be attached to one of asubstrate table and a reference frame of a lithographic apparatus andthe sensor may be attached to the other of the substrate table andreference frame. The sensor senses radiation redirected by the gratingto measure the relative position between the substrate table and thereference frame.

SUMMARY

The presence of contamination (e.g., a particle, a droplet of liquid,etc.) on the sensor or grating of the above positional measurementdevice can be a problem. This is because the contamination can interferewith the radiation impinging on or redirected by the grating/sensor andthereby cause a false reading. This is a particular difficulty inimmersion lithography systems where splashing of liquid may occurresulting in a chance of a droplet falling on the grating or sensor.Because of the proximity of the grating and sensor to where liquid ishandled in an immersion apparatus, it is likely that if measures are nottaken to mitigate the circumstances, a droplet of liquid will land onthe sensor or grating and cause measurement results to be faulty.

It is desirable, for example, to provide a lithographic apparatus inwhich the chances of a false measurement in a grating and sensorpositional measurement device being made are reduced.

According to an aspect of the invention, there is provided alithographic apparatus comprising: a substrate table configured to holda substrate; a reference frame; a grating attached to the substratetable or the reference frame; a sensor attached to the other of thesubstrate table or the reference frame, the sensor configured to detectradiation redirected by the grating to measure the relative positionbetween the substrate table and the reference frame; and a barrierassociated with the grating, the barrier positioned to hindercontamination from reaching the grating and having a surface facing awayfrom the grating at a distance between 300 μm and 5 mm from the grating.

According to an aspect of the invention, a lithographic apparatuscomprising: a substrate table configured to hold a substrate; areference frame; a grating attached to the substrate table or thereference frame; a sensor attached to the other of the substrate tableor the reference frame, the sensor configured to detect radiationredirected by the grating to measure the relative position between thesubstrate table and the reference frame; and a barrier associated withthe grating or sensor, the barrier positioned to hinder contaminationfrom reaching the associated grating or sensor and having a surfacefacing away from the associated grating or sensor at a distance between300 μm and 5 mm from the associated grating or sensor.

In an embodiment, the surface of the barrier is liquidphobic or has aliquidphobic coating. In an embodiment, the barrier is integral with thegrating. In an embodiment, a plate forms the barrier and the grating isformed on a backside of the plate. In an embodiment, the barriercomprises a plate. In an embodiment, the barrier is in contact with thegrating or the sensor. In an embodiment, the barrier is positioneddistal from the associated grating or sensor such that a gap is presentbetween the barrier and associated grating or sensor. In an embodiment,the apparatus further comprises a gas supply configured to provide a gasto the gap. In an embodiment, the gas supply is a conditioned gas supplysuch that gas with a certain temperature can be supplied to the gap. Inan embodiment, the apparatus further comprises a frame, wherein thebarrier is attached to the frame. In an embodiment, the barriersurrounds a space adjacent the associated grating or sensor. In anembodiment, the apparatus further comprises a hole in the barrier toallow equalization of pressure of gas in the space and outside of thespace. In an embodiment, the barrier comprises a polymer, desirably afluoropolymer such as PTFE. In an embodiment, the barrier is between 0.1and 2 μm thick, desirably less than 1 μm thick. In an embodiment, thebarrier comprises a plurality of separate barriers each being positionedadjacent a different portion of the associated grating or sensor. In anembodiment, the grating comprises a plurality of lines on a surface. Inan embodiment, the lines are lines of chromium. In an embodiment, thebarrier is removably attached to the apparatus. In an embodiment, thebarrier, in use, is at an angle to horizontal such that any liquiddroplets on the barrier move under the influence of gravity. In anembodiment, the apparatus further comprises a contamination removaldevice configured to remove contamination from the surface of thebarrier. In an embodiment, the distance is desirably greater than 500μm, more desirably greater than 1 mm. In an embodiment, the distance isdesirably less than 4 mm, more desirably less than 3 mm.

According to an aspect of the invention, there is provided alithographic apparatus comprising: a substrate table configured to holda substrate; a reference frame; a grating attached to the substratetable or the reference frame; a sensor attached to the other of thesubstrate table or the reference frame, the sensor configured to detectradiation redirected by the grating to measure the relative positionbetween the substrate table and the reference frame; and a contaminationremoval device configured to remove contamination from a surface bychanging a property of the surface, the surface being a surface of (i)the sensor, or (ii) the grating, or (iii) a barrier at least partlycovering the sensor or grating, or (iv) any combination selected from(i)-(iii).

In an embodiment, the contamination removal device is configured toremove particles and/or droplets from the surface. In an embodiment, theproperty is surface topography or electrostatic potential. In anembodiment, the contamination removal device comprises a transducerconfigured to induce surface acoustic waves into the surface. In anembodiment, the contamination removal device comprises a plurality ofelectrodes formed on or in the surface and a voltage applicatorconfigured to apply voltage to the electrodes.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising projecting a patterned beam of radiationonto a substrate held on a substrate table, wherein a position of thesubstrate table relative to the projection system is measured using agrating and a sensor, wherein a barrier is positioned to hindercontamination from reaching the grating or sensor, the barrier having asurface facing away from the associated grating or sensor at a distancebetween 300 μm and 5 mm from the associated grating or sensor.

According to an aspect of the invention, there is provided a method ofremoving a particle and/or droplet on or preventing a particle and/ordroplet from adhering to a surface of a grating, a sensor or a barrierat least partly covering the sensor or the grating, the methodcomprising changing a property of the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts a further liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a further liquid supply system for use in a lithographicprojection apparatus;

FIG. 6 is a schematic illustration, in cross-section, of a grating andsensor positional measurement device of an immersion lithographicapparatus according to an embodiment of the present invention;

FIG. 7 depicts, in plan, the apparatus of FIG. 6;

FIG. 8 depicts, in perspective view, a substrate table of an embodimentof the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or DUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device MA inaccordance with certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate W inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system IL may include various types of opticalcomponents, such as refractive, reflective, magnetic, electromagnetic,electrostatic or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The support structure MT holds the patterning device MA. It holds thepatterning device MA in a manner that depends on the orientation of thepatterning device MA, the design of the lithographic apparatus, andother conditions, such as for example whether or not the patterningdevice MA is held in a vacuum environment. The support structure MT canuse mechanical, vacuum, electrostatic or other clamping techniques tohold the patterning device MA. The support structure MT may be a frameor a table, for example, which may be fixed or movable as required. Thesupport structure MT may ensure that the patterning device MA is at adesired position, for example with respect to the projection system PS.Any use of the terms “reticle” or “mask” herein may be consideredsynonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device MA may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted minorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more patterning device tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source SO and the lithographic apparatus may beseparate entities, for example when the source SO is an excimer laser.In such cases, the source SO is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source SO may be an integral part of thelithographic apparatus, for example when the source SO is a mercurylamp. The source SO and the illuminator IL, together with the beamdelivery system BD if required, may be referred to as a radiationsystem.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as n-outer andn-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator IL can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator IL may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section. Similar to the source SO, the illuminator IL may or maynot be considered to form part of the lithographic apparatus. Forexample, the illuminator IL may be an integral part of the lithographicapparatus or may be a separate entity from the lithographic apparatus.In this latter case, the lithographic apparatus may be configured toallow the illuminator IL to be mounted thereon and optionally detachableand may be, for example, separately provided by the lithographicapparatus manufacturer or another supplier.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device MA. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam B is projected onto a target portion C at one time (i.e.a single static exposure). The substrate table WT is then shifted in theX and/or Y direction so that a different target portion C can beexposed. In step mode, the maximum size of the exposure field limits thesize of the target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam Bis projected onto a target portion C (i.e. a single dynamic exposure).The velocity and direction of the substrate table WT relative to thesupport structure MT may be determined by the (de-) magnification andimage reversal characteristics of the projection system PS. In scanmode, the maximum size of the exposure field limits the width (in thenon-scanning direction) of the target portion C in a single dynamicexposure, whereas the length of the scanning motion determines theheight (in the scanning direction) of the target portion C.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam B is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Arrangements for providing liquid between a final element of theprojection system and the substrate can be classed into at least twogeneral categories. These are the bath type arrangement in whichsubstantially the whole of the substrate and optionally part of thesubstrate table is submersed in a bath of liquid and the so calledlocalized immersion system which uses a liquid supply system in whichliquid is only provided to a localized area of the substrate. In thelatter category, the space filled by liquid is smaller in plan than thetop surface of the substrate and the area filled with liquid remainssubstantially stationary relative to the projection system while thesubstrate moves underneath that area.

A further arrangement, to which an embodiment of the present inventionis directed, is the all wet solution in which the liquid is unconfined.In this arrangement substantially the whole top surface of the substrateand all or part of the substrate table is covered in immersion liquid.The depth of the liquid covering at least the substrate is small. Theliquid may be a film, such as a thin film, of liquid on the substrate.Any of the liquid supply devices of FIGS. 2-5 may be used in such asystem; however, sealing features are not present, are not activated,are not as efficient as normal or are otherwise ineffective to sealliquid to only the localized area. Four different types of localizedliquid supply systems are illustrated in FIGS. 2-5. The liquid supplysystems disclosed in FIGS. 2-4 were described above.

Another arrangement which has been proposed is to provide the liquidsupply system with a liquid confinement structure which extends along atleast a part of a boundary of the space between the final element of theprojection system and the substrate table. Such an arrangement isillustrated in FIG. 5. The liquid confinement structure is substantiallystationary relative to the projection system in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). A seal is formed between the liquid confinementand the surface of the substrate. In an embodiment, a seal is formedbetween the liquid confinement structure and the surface of thesubstrate and may be a contactless seal such as a gas seal. Such asystem is disclosed in United States patent application publication no.US 2004-0207824.

FIG. 5 schematically depicts a localized liquid supply system or fluidhandling structure with a barrier member or fluid confinement structure12, which extends along at least a part of a boundary of the space 11between the final element of the projection system PS and the substratetable WT or substrate W. (Please note that reference in the followingtext to surface of the substrate W also refers in addition or in thealternative to a surface of the substrate table WT, unless expresslystated otherwise.) The fluid confinement structure 12 is substantiallystationary relative to the projection system PS in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). In an embodiment, a seal is formed between thefluid confinement structure 12 and the surface of the substrate W andmay be a contactless seal such as a gas seal or fluid seal.

The fluid confinement structure 12 at least partly contains liquid inthe space 11 between a final element of the projection system PS and thesubstrate W. A contactless seal, such as a gas seal 16, to the substrateW may be formed around the image field of the projection system PS sothat liquid is confined within the space 11 between the substrate Wsurface and the final element of the projection system PS. The space 11is at least partly formed by the fluid confinement structure 12positioned below and surrounding the final element of the projectionsystem PS. Liquid is brought into the space 11 below the projectionsystem PS and within the fluid confinement structure 12 by liquid inlet13. The liquid may be removed by liquid outlet 13. The fluid confinementstructure 12 may extend a little above the final element of theprojection system PS. The liquid level rises above the final element sothat a buffer of liquid is provided. In an embodiment, the fluidconfinement structure 12 has an inner periphery that at the upper endclosely conforms to the shape of the projection system PS or the finalelement thereof and may, e.g., be round. At the bottom, the innerperiphery closely conforms to the shape of the image field, e.g.,rectangular, though this need not be the case.

The liquid is contained in the space 11 by the gas seal 16 which, duringuse, is formed between the bottom of the fluid confinement structure 12and the surface of the substrate W. The gas seal 16 is formed by gas,e.g, air or synthetic air but, in an embodiment, N₂ or another inertgas. The gas in the gas seal 16 is provided under pressure via inlet 15to the gap between fluid confinement structure 12 and substrate W. Thegas is extracted via outlet 14. The overpressure on the gas inlet 15,vacuum level on the outlet 14 and geometry of the gap are arranged sothat there is a high-velocity gas flow inwardly that confines theliquid. The force of the gas on the liquid between the fluid confinementstructure 12 and the substrate W contains the liquid in a space 11. Theinlets/outlets may be annular grooves which surround the space 11. Theannular grooves may be continuous or discontinuous. The flow of gas iseffective to contain the liquid in the space 11. Such a system isdisclosed in United States patent application publication no. US2004-0207824.

An embodiment of the present invention can be applied to any type oflithographic apparatus, particularly an immersion apparatus.

The example of FIG. 5 is a so called localized area arrangement in whichliquid is only provided to a localized area of the top surface of thesubstrate W at any one time. Other arrangements are possible, includingfluid handling systems which make use of a single phase extractor(whether or not it works in two phase mode) as disclosed, for example,in United States patent application publication no US 2006-0038968. Inan embodiment, a single phase extractor may comprise an inlet which iscovered in a porous material which is used to separate liquid from gasto enable single-liquid phase liquid extraction. A chamber downstream ofthe porous material is maintained at a slight under pressure and isfilled with liquid. The under pressure in the chamber is such that themeniscuses formed in the holes of the porous material prevent ambientgas from being drawn into the chamber. However, when the porous surfacecomes into contact with liquid there is no meniscus to restrict flow andthe liquid can flow freely into the chamber. The porous material has alarge number of small holes, e.g. of diameter in the range of 5 to 50μm. In an embodiment, the porous material is at least slightlyliquidphilic (e.g., hydrophilic), i.e. having a contact angle of lessthan 90° to the immersion liquid, e.g. water. Another arrangement whichis possible is one which works on a gas drag principle. The so-calledgas drag principle has been described, for example, in United Statespatent application publication no. US 2008-0212046 and U.S. patentapplication No. 61/071,621 filed on 8 May 2008. In that system theextraction holes are arranged in a shape which desirably has a corner.The corner may be aligned with the stepping and scanning directions.This reduces the force on the meniscus between two openings in thesurface of the fluid handing structure for a given speed in the step orscan direction compared to if the two outlets were aligned perpendicularto the direction of scan. An embodiment of the invention may be appliedto a fluid handling structure used in all wet immersion apparatus. Inthe all wet embodiment, fluid is allowed to cover the whole of the topsurface of the substrate table, for example, by allowing liquid to leakout of a confinement structure which confines liquid to between thefinal element of projection system and the substrate. An example of afluid handling structure for an all wet embodiment can be found in U.S.patent application No. 61/136,380 filed on 2 Sep. 2008.

FIG. 6 shows, in cross-section, an immersion lithographic apparatuswhich uses a grating and sensor positional measurement device. Anembodiment of the invention is equally applicable to a non-immersionlithographic apparatus.

FIG. 6 illustrates schematically, in cross-section, a substrate table WTand a liquid supply system 12 as well as a projection system PS. As canbe seen, the apparatus depicted is a so called all wet system in which afilm of liquid covers the whole of the top surface of the substratetable WT. However, an embodiment of the invention is equally applicableto other types of apparatus.

In order to determine the position of the substrate table WT relative tothe projection system PS, a grating and sensor positional measurementdevice is used. In the embodiment of FIG. 6 the sensor 20 is attached tothe substrate table WT and the grating 50 is attached to a referenceframe RF, which may be a so called metrology frame. The relativeposition of the grating 50 to the projection system PS is known andremains substantially constant because the position of the projectionsystem PS relative to the reference frame RF is known. The referenceframe RF of a lithographic apparatus may be mounted with passive oractive gas-mounts on a base frame BF (referring to FIG. 1) to filter anyexternal disturbances such as vibrations in the factory floor. In thisway, the optical column of the projection system PS may be held in asubstantially stationary position. During a scanning movement of thesubstrate table WT, it is desired to know the position of the substratetable WT with respect to the optical column. Therefore, the positionalmeasurement device is provided to determine the position of thesubstrate table WT with respect to the reference frame RF. In oneembodiment, the sensor 20 is attached to the reference frame RF whilethe grating 50 is formed on or attached to the substrate table WT. Suchan embodiment is described with reference to FIG. 8 below.

The grating 50 comprises a plurality of lines or spots, for examplechromium lines or spots, on a surface. The surface could be the surfaceof a plate 55, for example. Together the plate 55 and grating 50 form agrid plate.

The plate 55 is desirably made of a low coefficient of thermal expansionmaterial. For example, a low coefficient of thermal expansion glass,glass ceramic or ceramic may be used such as Zerodur™.

The lines of the grating 50 are, for example, a plurality of parallellines (1 dimensional). However, another form of grating 50 could beused. For example, the grating 50 could be a plurality of lines in afirst direction as well as a plurality of lines in a second directionwherein the first and second directions are substantially perpendicular,for example (2 dimensional).

A beam of radiation is directed towards the grating 50, for example, bya radiation source 30. In one embodiment the radiation source 30produces light of a wavelength of about 600 nm, for example. In oneembodiment the radiation source 30 produces light of a wavelength ofabout 780 nm. However, the exact wavelength is not critical. Theradiation source is attached to the reference frame RF. The radiationsource 30 may also or instead be attached to the substrate table WT. Thebeam of radiation directed to the grating 50 by the radiation source 30is reflected and/or diffracted by the grating 50 and this radiation isthen detected by the sensor 20. Together the sensor 20 and radiationsource 30 form an encoder 40.

Positional measurement is carried out by measuring the position of thesensor 20 with respect to the grating 50 in one or more degrees offreedom. As is illustrated in FIG. 7, which is a plan view of the systemof FIG. 6, a plurality of sensors 20 and radiation sources 30 areattached to the substrate table WT. Each encoder 40 is capable ofmeasuring a position of the substrate table WT in two degrees offreedom, making position measurement in six degrees of freedom possible.Using the encoder-type measurement system a position measurement withhigh accuracy is possible. Any other suitable configuration of encoderheads may be applied. In the embodiment of FIG. 7 there are at leastthree encoders on the substrate table WT and desirably four. This allowstranslational and rotational movement of the substrate table WT to bedetected. Also, as illustrated in FIG. 7, the grating 50 is split intofour separate gratings 50 a, 50 b, 50 c, 50 d. The lines of each grating50 a, 50 b, 50 c, 50 d may have any orientation with respect to eachother. In one embodiment the lines of each grating 50 a, 50 b, 50 c, 50d are non parallel and non orthogonal with respect to each other.

The grating 50 comprises a central hole for the optical column of theprojection system PS and is mounted on the reference frame RF with anumber of mounting devices.

The presence of a droplet of liquid (for example immersion liquid) or ofa contaminant particle on the grating 50 can lead to measurement errors.This is because the radiation beam from the radiation source 30 eitherbefore or after it is redirected by the grating 50 can have one or moreof its properties (e.g. direction of travel, direction of polarization,intensity) changed thereby leading to a measurement error when thesensor 20 detects the radiation which has had its one or more propertieschanged (i.e. interfered with).

In an embodiment of the present invention, a barrier 100 is used. Thebarrier 100 is positioned relative to the grating 50 so as to hindercontamination (e.g., a droplet and/or a particle) from reaching theassociated grating 50.

The barrier 100 has a surface 110 which faces away from the grating 50.Any contamination which adheres to the barrier 100 will adhere to thatsurface 110. The surface 110 is arranged such that any contaminationwhich adheres to it will be far enough away from a focal plane at whicha radiation beam from the radiation source 30 focuses on the grating 50so that the contamination does not interfere with the measurement. Thatis, the contamination will be out of focus to the encoder 40 comprisingthe sensor 20 and radiation source 30.

In this way, even though the contamination is present, it will notsubstantially interfere with the reading made by the sensor positionalmeasurement system.

A maximum droplet size which can adhere onto the surface 110 (taking,for example, water as the immersion liquid) is about 1.8 mm in diameter.This is for a surface 110 with a static contact angle of 0°. At a staticcontact angle of 80°, the maximum droplet size is more like 0.6 mm indiameter. For a static contact angle of 140°, the maximum dropletdiameter is about 0.1 mm. Based on this knowledge it is possible tocalculate a range of distances of the surface 110 from the grating 50 atwhich most droplets will be out of focus as described above. It islikely that any contaminant particles will be of a similar size. As willbe clear from the below, any smaller droplets or particles willautomatically be out of focus if the surface 110 is far enough forlarger droplets or particles to be out of focus.

An equation can be written which determines the distance the surface 110should be from the grating 50 in order for the contamination to be outof focus. The equation is distance>diameter of contamination/tan(θ)wherein θ is the opening angle of the optics of the sensor 20. A typicalopening angle θ is 10°. Therefore, for a contaminant diameter of about100 μm, the distance between the surface 110 and the grating 50 shouldbe at least 567 μm. For a contaminant of about 0.6 mm diameter, thedistance should be at least 3 mm. There are practical limitations to thedistance. There is only a given amount of space available for thebarrier 100. If the barrier 100 is too far from the grating 50, it getsclose to the top of the substrate table WT and is thereby more likely tobe contaminated (e.g., splashed with liquid). If the distance is toolow, large contaminants will interfere with the radiation beam from theradiation source 30. Therefore, in an embodiment of the presentinvention, the distance is desirably between 300 μm and 5 mm. Such adistance is suitable to ensure that contaminants of a size up to about0.7 mm (which are the likely biggest sizes) do not affect the positionalmeasurement reading. In one embodiment, the distance is less than 4 mm.In an embodiment, the distance is less than 3 mm. In an embodiment, thedistance is more than 500 μm, or more than 1 mm.

In one embodiment, the surface 110 is between 300 μm and 3 mm away fromthe grating 50. This should be far enough so that any contaminant willbe out of focus to the encoder 40 comprising the sensor 20 and radiationsource 30 but not such a great distance that it would result in thesurface 110 being more likely to attract more contamination (because itis closer to the substrate table WT and therefore, e.g., immersionliquid).

The barrier 100 helps ensures that any stray contaminants are out offocus to the sensor 20 and thereby do not disrupt the reading. Further,when a droplet dries, it may leave behind a drying stain. Any suchdrying stain would also be out of focus. The barrier 100 is cheaper andsimpler to replace or clean than a grating 50.

The barrier 100 could be made of any suitable material. A material usedfor a pellicle to protect a patterning device is suitable. For example,a fluoropolymer, such as PTFE, may be suitable. In an embodiment thebarrier 100 may be a flexible polymer. In an embodiment the material isliquidphobic. For example, the immersion liquid may make a staticcontact angle (at room temperature and at atmospheric pressure) with thematerial of greater than 90°, desirably greater than 100°, moredesirably greater than 110°, more desirably greater than 120, 130, 140or 150°. The higher the contact angle, the smaller the maximum dropletsize which can adhere to the barrier 100. In one embodiment the staticcontact angle is less than 180° . In one embodiment the barrier 100 maybe 0.7 μm thick, desirably between 0.2 μm and 1.5 μm thick. Otherpolymers may be suitable. The material of the barrier 100 issufficiently transparent to the wavelength of radiation from theradiation source 30 such that a meaningful signal can be detected by thesensor 20.

Other materials for the barrier 100 may be suitable. For example, quartzor Zerodur™ may be suitable. In this case the barrier 100 is relativelystiff. In this embodiment the barrier 100 may be made thicker, forexample between 0.1 and 0.3 mm thick. In one embodiment, the barrier 100may be comprised of a relatively thick plate (1-4 mm) which is attacheddirectly to the plate 55 of the grating 50, with or without a gapbetween the barrier 100 and the grating 50. An advantage of a hard orstiff or thick barrier 100 is that the barrier 100 may provide someprotection to the grating 50 in the case of a collision.

In the embodiment of FIG. 6 a gap 120 exists between the grating 50 andthe barrier 100. In one embodiment a conditioned gas supply 130 may beprovided. The conditioned gas supply 130 provides gas of a certaintemperature to the gap 120. This helps avoid temperature fluctuations inthe grating 50 which could otherwise lead to positional measurementerrors. A conditioning device (not shown in FIG. 6) is present on theother side of the plate 55 for the same purpose.

A frame 140 is provided to which the barrier 100 may be attached. Thebarrier 100 may be detachable from the frame 140. This helps inreplacement and/or cleaning of the barrier 100. In one embodiment,instead of attaching the barrier 100 to the plate 55 of the grating 50,the barrier 100 could be attached directly to the reference frame RF.

In one embodiment no gap 120 is provided between the barrier 100 and thegrating 50. The barrier 100 and grating 50 may be integral. For example,the grating 50 may be defined on the surface of the plate 55 facing awayfrom the sensor 20 and radiation source 30. That is, any lines or spotswhich make up the grating 50 are formed on the surface of the plate 55facing away from the sensor 20. In this way, any contamination would bepresent on a surface of the plate 55 which is a distance away from thegrating 50. Thus, the plate 55 itself would form the barrier 100 and thesurface on which any contamination could adhere would be at a distancefrom the grating 50 so that any contamination on the surface would beout of focus to the sensor 20.

In one embodiment the barrier 100 is made of a material and/or has acoating which is liquidphobic to the immersion liquid. In an embodimentthe material is liquidphobic. For example, the immersion liquid may makea static contact angle with the material of between 90 and 180°,desirably greater than 100°, more desirably greater than 110° or greaterthan 120°. This can help in preventing droplets from adhering to thesurface 110 and/or make them easier to be removed from the surface 110.

It may be necessary from time to time (for example, every day, each lot,or each substrate) to clean the surface 110 of the barrier 100. One wayof doing this would be to provide a gas knife 150 to blow a stream ofgas onto the surface 110 and thereby remove contamination (e.g.,particles and/or droplets).

An additional or alternative way of removing contamination is to use acontamination removal device 160 which removes contamination (e.g.,particles and/or droplets) by changing a property of the surface 110.For example, surface acoustic waves (SAW) could be introduced to thesurface 110 by a transducer configured to induce surface acoustic waves.This varies the surface topography of the surface 110 with time (i.e.surface waves are introduced into it). This change in surface topographyof the surface 110 (i.e. shape of the surface, which is a property ofthe surface) with time can help in the removal of contamination. Byproperly positioning and configuring the SAW generator, the acousticwaves may remove to a large extent droplets, chemical contaminationand/or particles present on the lower surface of the barrier 100.Besides this, if the SAW generator is activated, it may prevent thesedroplets, chemical contamination and/or particles to ever attachthemselves to the surface 110 in the first place. It will be appreciatedit may not always be possible (or even needed) to have the SAW generatoractivated, as the acoustic waves in the barrier 100 may have a negativeinfluence on the positional measurement and/or motion of the substratetable WT.

It may be possible to make sure the acoustic waves move through thebarrier 100 in such a way that contamination attached to the barrier 100is moved over the surface 110 of the barrier 100 to a location wherethey can do less harm, or can effectively be removed. Besidesconfiguring the acoustic wave generator itself, tilting the barrier 100a bit may help.

Alternatively or additionally, a plurality of electrodes could be formedon the surface 110 and by applying a voltage to the electrodes theelectrostatic potential of the surface 110 (a property of the surface110) could be varied. Such a variation in electrostatic potential can beused to move contamination to the edge of the surface 110 where it couldbe collected or at least where it will not interfere with the radiationbeam from the radiation source 30. Electrostatic moving of droplets isexplained in greater detail in U.S. patent application No. 60/996,736,filed on 3 Dec. 2007.

In an embodiment, where a barrier 100 of a flexible polymer is used, itmay not be possible to induce surface acoustic waves. However, in thiscase (as well as in others) it may be possible to shake the barrier 100and thereby remove any adhering contamination.

In the case of flexible polymer being used as the barrier, it may bepossible to have a supply roll of polymer at one end and a collectingroll at the other end. The polymer is unrolled from the supply roll andwound up at the other end to expose fresh polymer. This can be done on aregular or even constant basis. Alternatively it can be done once thesurface 110 of a given length of polymer has become too contaminated forthe positional measurement device to operate with the desired degree ofaccuracy.

Although in relation to FIG. 6 the barrier 100 and various contaminationremoval devices have been described with relation to the grating 50, allor some of these devices could equally well alternatively oradditionally be applied to the sensor 20 or the radiation source 30. Forexample, if droplets land on the sensor 20 or the radiation source 30,the signal will not affect the measured position but may attenuate thesignal. Therefore, an error in positional result will not occur(contrary to the case where if that droplet is on the grating 50), butno position at all may be measured because the signal strength is toolow. In one embodiment, no barrier 100 is present. Instead, at least oneparticle removal device 160, 170 which changes a property of the surfaceof the grating 50 or the surface of the sensor 20 or radiation source 30is/are present.

In one embodiment the barrier 100 is wrapped around the frame 140 suchthat a space adjacent the grating 50 is enclosed by the grating 50 andthe barrier 100. This helps prevent particles from finding their wayonto the grating 50 at the edges of the barrier 100. However, in thiscase it may be necessary to equalize pressure in the gap 120 and outsideof the gap 120. To achieve this one or more holes 111 can be provided inone or more edges of the barrier 100 which one or more edges aresubstantially perpendicular to the plane of the surface of the grating50.

A labyrinth seal (i.e. a seal which seals by presenting a tortuous pathfrom one end to the other) may be provided around such one or more holesto help prevent contamination entering the gap 120. In one embodiment, afilter may be used to filter any gas passing through such one or moreholes into the gap 120.

In one embodiment the barrier 100 is arranged to be at an angle tohorizontal. In this way any contamination (e.g., droplets) may move toone edge of the barrier 100 where it may be collected under theinfluence of gravity or where it may remain.

FIG. 7 illustrates, in plan, the arrangement of FIG. 6. In FIG. 7 fourgrid plates 55 a, 55 b, 55 c, 55 d are illustrated which surround theprojection system PS. The dimensions of the grid plates are chosen suchthat at least three (3) encoders 40 on the substrate table WT arepositioned under a grid plate 55 at any one time for any position ofsubstrate table WT (at which the position is to be measured by the gridplate/sensor positional detector).

Each grating 50 could be provided with one or more barriers 100 so thatthe entire surface of the grating 50 is covered by a bather 100.Desirably only one barrier 100 is used per grating 50 so that none ofthe grating 50 is covered by a frame.

FIG. 8 illustrates an embodiment in which the grid plate 55 or thegrating 50 is formed on a top surface of the substrate table WTsurrounding a substrate W. The barrier 100 is omitted from FIG. 8 forclarity. In this case the encoder 40 comprising the sensor 20 andradiation source 30 is attached to the reference frame RF. In this case,as with the embodiment of FIG. 6, the barrier could be provided over thegrating 50 and/or over the sensor 20 and/or over the radiation source30. The various contamination removal devices can be used on the grating50, the sensor 20, the radiation source 30 and/or a barrier 100 over anyone of those objects, as in the embodiment of FIG. 6.

As will be appreciated, any of the above described features can be usedwith any other feature and it is not only those combinations explicitlydescribed which are covered in this application.

In an aspect, there is provided there is provided a lithographicapparatus comprising: a substrate table configured to hold a substrate;a reference frame; a grating attached to the substrate table or thereference frame; a sensor attached to the other of the substrate tableor the reference frame, the sensor configured to detect radiationredirected by the grating to measure the relative position between thesubstrate table and the reference frame; and a barrier associated withthe grating, the barrier positioned to hinder contamination fromreaching the grating and having a surface facing away from the gratingat a distance between 300 μm and 5 mm from the grating. Optionally, thesurface of the barrier is liquidphobic or has a liquidphobic coating.Optionally, the barrier is positioned distal from the associated gratingor sensor such that a gap is present between the barrier and associatedgrating or sensor. Desirably, the apparatus further comprises a gassupply to provide a gas to the gap. Desirably, the gas supply is aconditioned gas supply such that gas with a certain temperature can besupplied to the gap. Desirably, the apparatus further comprises a frame,wherein the barrier is attached to the frame. Desirably, the barriersurrounds a space adjacent the associated grating or sensor. Desirably,the apparatus further comprises a hole in the barrier to allowequalization of pressure of gas in the space and outside of the space.Optionally, the barrier comprises a polymer, desirably a fluoropolymersuch as PTFE. Optionally, the bather is between 0.1 and 2 μm thick,desirably less than 1 μm thick. Optionally, the barrier is integral withthe grating. Desirably, a plate forms the barrier and the grating isformed on a backside of the plate. Optionally, the barrier comprises aplate. Optionally, the barrier is in contact with the grating or thesensor. Optionally, the barrier comprises a plurality of separatebarriers each being positioned adjacent a different portion of theassociated grating or sensor. Optionally, the grating comprises aplurality of lines on a surface. Desirably, the lines are lines ofchromium. Optionally, the barrier is removably attached to theapparatus. Optionally, the barrier, in use, is at an angle to horizontalsuch that any liquid droplets on the barrier move under the influence ofgravity. Optionally, the apparatus further comprises a contaminationremoval device to remove contamination from the surface of the barrier.Optionally, the distance is desirably greater than 500 μm, moredesirably greater than 1 mm. Optionally, the distance is desirably lessthan 4 mm, more desirably less than 3 mm.

In an aspect, there is provided a lithographic apparatus comprising asubstrate table configured to hold a substrate; a reference frame; agrating attached to the substrate table or the reference frame; a sensorattached to the other of the substrate table or the reference frame, thesensor configured to detect radiation redirected by the grating tomeasure the relative position between the substrate table and thereference frame; and a contamination removal device configured to removecontamination from a surface by changing a property of the surface, thesurface being a surface of (i) the sensor, or (ii) the grating, or (iii)a barrier at least partly covering the sensor or grating, or (iv) anycombination selected from (i)-(iii). Optionally, the contaminationremoval device is configured to remove particles and/or droplets fromthe surface. Optionally, the property is surface topography orelectrostatic potential. Optionally, the contamination removal devicecomprises a transducer to induce surface acoustic waves into thesurface. Optionally, the contamination removal device comprises aplurality of electrodes formed on or in the surface and a voltageapplicator to apply voltage to the electrodes.

In an aspect, there is provided a device manufacturing method comprisingprojecting a patterned beam of radiation onto a substrate held on asubstrate table, wherein a position of the substrate table relative tothe projection system is measured using a grating and a sensor, whereina barrier is positioned to hinder contamination from reaching thegrating or sensor, the barrier having a surface facing away from theassociated grating or sensor at a distance between 300 μm and 5 mm fromthe associated grating or sensor.

In an aspect, there is provided a method of removing a particle and/ordroplet on or preventing a particle and/or droplet from adhering to asurface of a grating, a sensor or a barrier at least partly covering thesensor or the grating, the method comprising changing a property of thesurface.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm). The term“lens”, where the context allows, may refer to any one or combination ofvarious types of optical components, including refractive and reflectiveoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the embodiments of the invention maytake the form of a computer program containing one or more sequences ofmachine-readable instructions describing a method as disclosed above, ora data storage medium (e.g. semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein. Further, themachine readable instruction may be embodied in two or more computerprograms. The two or more computer programs may be stored on one or moredifferent memories and/or data storage media.

The controllers described herein may each or in combination be operablewhen the one or more computer programs are read by one or more computerprocessors located within at least one component of the lithographicapparatus. The controllers may each or in combination have any suitableconfiguration for receiving, processing, and sending signals. One ormore processors are configured to communicate with the at least one ofthe controllers. For example, each controller may include one or moreprocessors for executing the computer programs that includemachine-readable instructions for the methods described above. Thecontrollers may include data storage medium for storing such computerprograms, and/or hardware to receive such medium. So the controller(s)may operate according the machine readable instructions of one or morecomputer programs.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath, only on a localized surface area of the substrate, or isunconfined. In an unconfined arrangement, the immersion liquid may flowover the surface of the substrate and/or substrate table so thatsubstantially the entire uncovered surface of the substrate table and/orsubstrate is wetted. In such an unconfined immersion system, the liquidsupply system may not confine the immersion fluid or it may provide aproportion of immersion liquid confinement, but not substantiallycomplete confinement of the immersion liquid.

A liquid supply system as contemplated herein should be broadlyconstrued. In certain embodiments, it may be a mechanism or combinationof structures that provides a liquid to a space between the projectionsystem and the substrate and/or substrate table. It may comprise acombination of one or more structures, one or more fluid openingsincluding one or more liquid openings, one or more gas openings or oneor more openings for two phase flow. The openings may each be an inletinto the immersion space (or an outlet from a fluid handling structure)or an outlet out of the immersion space (or an inlet into the fluidhandling structure). In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising; a substrate table configured tohold a substrate; a reference frame; a grating attached to the substratetable or the reference frame; a sensor attached to the other of thesubstrate table or the reference frame, the sensor configured to detectradiation redirected by the grating to measure the relative positionbetween the substrate table and the reference frame; and a hairierassociated with the grating or sensor, the bather positioned to hindercontamination from reaching the associated grating or sensor and havinga surface facing away from the associated grating or sensor at adistance between 300 vim and 5 mm from the associated grating or sensor.2. The apparatus of claim 1, wherein the surface of the barrier isliquidphobic or has a liquidphobic coating.
 3. The apparatus of claim 1,wherein the barrier is integral with the grating.
 4. The apparatus ofclaim 1, wherein the harrier comprises a plate.
 5. The apparatus ofclaim 1, wherein the barrier is in contact with the grating or thesensor.
 6. The apparatus of claim 1, wherein the barrier is positioneddistal from the associated grating or sensor such that a gap is presentbetween the barrier and associated grating or sensor.
 7. The apparatusof claim 6, further comprising a gas supply configured to provide a gasto the gap.
 8. The apparatus of claim 6, further comprising a frame,wherein the barrier is attached to the frame.
 9. The apparatus of claim6, wherein the barrier surrounds a space adjacent the associated gratingor sensor.
 10. The apparatus of claim 9, further comprising a hole inthe barrier to allow equalization of pressure of gas in the space andoutside of the space.
 11. The apparatus of claim 1, wherein the barrieris between 0.1 and 2 μm thick.
 12. The apparatus of claim 1, wherein thebarrier comprises a plurality of separate barriers each being positionedadjacent a different portion of the associated grating or sensor. 13.The apparatus of claim 1, wherein the barrier is removably attached tothe apparatus.
 14. The apparatus of claim 1, wherein the barrier, inuse, is at an angle to horizontal such that any liquid droplets on thebarrier move under the influence of gravity.
 15. The apparatus of claim1, further comprising a contamination removal device configured toremove contamination from the surface of the barrier.
 16. A lithographicapparatus comprising: a substrate table configured to hold a substrate;a reference frame; a grating attached to the substrate table or thereference frame; a sensor attached to the other of the substrate tableor the reference frame, the sensor configured to detect radiationredirected by the grating to measure the relative position between thesubstrate table and the reference frame; and a contamination removaldevice configured to remove contamination from a surface by changing aproperty of the surface, the surface being a surface of (i) the sensor,or (ii) the grating, or (iii) a barrier at least partly covering thesensor or grating, or (iv) any combination selected from (i)-(iii). 17.The apparatus of claim 16, wherein the property is surface topography orelectrostatic potential.
 18. The apparatus of claim 16, wherein thecontamination removal device comprises a transducer configured to inducesurface acoustic waves into the surface.
 19. The apparatus of claim 16,wherein the contamination removal device comprises a plurality ofelectrodes formed on or in the surface and a voltage applicatorconfigured to apply voltage to the electrodes.
 20. A devicemanufacturing method comprising projecting a patterned beam of radiationonto a substrate held on a substrate table, wherein a position of thesubstrate table relative to the projection system is measured using agrating and a sensor, wherein a barrier is positioned to hindercontamination from reaching the grating or sensor, the barrier having asurface facing away from the associated grating or sensor at a distancebetween 300 μm and 5 mm from the associated grating or sensor.
 21. Amethod of removing a particle and/or droplet on or preventing a particleand/or droplet from adhering to a surface of a grating, a sensor or abarrier at least partly covering the sensor or the grating, the methodcomprising changing a property of the surface.